Adaptive safety pacing

ABSTRACT

Methods and systems involve adjusting an energy used for safety pacing based on the capture threshold. The safety pacing energy may be adjusted prior to a capture threshold test. During the capture threshold test, backup safety paces are delivered using the adjusted pacing energy. Following suspension of automatic capture verification, the device may enter a suspension mode. During the suspension mode, safety pacing pulses are delivered using a pacing energy adjusted based on capture threshold.

RELATED PATENT DOCUMENTS

This application is a continuation of U.S. patent application Ser. No.10/952,345 now U.S. Pat. No. 7,813,799, filed on Sep. 28, 2004, whichclaims the benefit of Provisional Patent Application Ser. No.60/527,780, filed on Dec. 8, 2003, to which Applicant claims priorityunder 35 U.S.C. §120 and 35 U.S.C. §119(e), respectively, and which areincorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates generally to implantable medical devicesand, more particularly, to implantable cardiac devices and methods foradaptively altering safety pacing.

BACKGROUND OF THE INVENTION

When functioning normally, the heart produces rhythmic contractions andis capable of pumping blood throughout the body. However, due to diseaseor injury, the heart rhythm may become irregular resulting in diminishedpumping efficiency. Arrhythmia is a general term used to describe heartrhythm irregularities arising from a variety of physical conditions anddisease processes. Cardiac rhythm management systems, such asimplantable pacemakers and cardiac defibrillators, have been used as aneffective treatment for patients with serious arrhythmias. These systemstypically comprise circuitry to sense electrical signals from the heartand a pulse generator for delivering electrical stimulation pulses tothe heart. Leads extending into the patient's heart are connected toelectrodes that contact the myocardium for sensing the heart'selectrical signals and for delivering stimulation pulses to the heart inaccordance with various therapies for treating the arrhythmias.

Cardiac rhythm management systems operate to stimulate the heart tissueadjacent to the electrodes to produce a contraction of the tissue.Pacemakers are cardiac rhythm management devices that deliver a seriesof low energy pace pulses timed to assist the heart in producing acontractile rhythm that maintains cardiac pumping efficiency. Pacepulses may be intermittent or continuous, depending on the needs of thepatient.

When a pace pulse produces a contraction in the heart tissue, theelectrical cardiac signal following the contraction is denoted thecaptured response. A pace pulse must exceed a minimum energy value, orcapture threshold, to produce a contraction. The capture threshold isdefined as the lowest pacing energy that consistently captures theheart. It is desirable for a pace pulse to have sufficient energy tostimulate capture of the heart without expending energy significantly inexcess of the capture threshold. Thus, accurate determination of thecapture threshold is required for efficient pace energy management. Ifthe pace pulse energy is too low, the pace pulses may not reliablyproduce a contractile response in the heart and may result inineffective pacing. If the pace pulse energy is too high, the patientmay experience discomfort and the battery life of the device will beshorter.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading thepresent specification, there is a need in the art for methods andsystems for automatically adjusting the pacing energy for pacing pulsesdelivered to the patient. The present invention fulfills these and otherneeds.

SUMMARY OF THE INVENTION

Embodiments of the invention are directed to methods and systems foradjusting an energy used for safety pacing pulses. One embodiment of theinvention involves a method for delivering safety pacing to a patient.The method includes determining a capture threshold and adjusting thesafety pacing energy based on the capture threshold.

In accordance with one aspect of the invention, adjustment of the safetypacing energy involves safety paces delivered during a capture thresholdtest. In accordance with another aspect, adjustment of the safety pacingenergy involves safety pacing pulses delivered when the device isoperating in a suspension mode after automatic capture verification hasbeen suspended. Adjustment of the safety pacing energy may involveadjusting the energy for safety pacing during automatic captureverification suspension to a predetermined amount above the capturethreshold.

Another embodiment of the invention is directed to a medical device fordelivering cardiac pacing therapy to a patient. The medical deviceincludes a plurality of electrodes configured to electrically couple toa heart and to deliver pacing pulses to a heart. Detection circuitry iscoupled to the electrodes and is configured to determine a capturethreshold energy associated with the pacing pulses. The medical devicealso includes a pulse generator coupled to the plurality of electrodesand the detection circuitry. The pulse generator is configured to adjustan energy for safety pacing based on the capture threshold energy.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of an adaptive safety pacing method in accordancewith embodiments of the present invention;

FIG. 2 is a partial view of an implantable medical device in accordancewith embodiments of the invention;

FIG. 3 is a block diagram of an implantable medical device in accordancewith embodiments of the invention;

FIG. 4 is a flowchart illustrating a method for delivering safety pacingupon suspension of automatic capture verification in accordance withembodiments of the invention; and

FIG. 5 is a flowchart illustrating a method of performing a capturethreshold test using an adjustable safety pacing energy in accordancewith embodiments of the invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings forming a part hereof, and inwhich are shown by way of illustration, various embodiments by which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

The present invention involves methods and systems for adjusting theenergy of safety pacing pulses. The safety pacing pulses comprise, forexample, safety backup pacing pulses delivered following the primarypulses of a capture verification test and safety pacing pulses deliveredafter automatic capture verification is suspended. In accordance withvarious embodiments described herein, the energy of the safety pacingpulses is adjusted based on previously acquired capture thresholdinformation.

FIG. 1 illustrates a method of adjusting the safety pacing energy inaccordance with embodiments of the invention. The capture threshold isdetermined 110, for example, through an automatic capture thresholdtesting procedure. The safety pacing energy is adjusted 120 based on thecapture threshold. The capture threshold of one heart chamber may beused to adjust the safety pacing energy of another heart chamber. Forexample, the system may determine the capture threshold of a first heartchamber, e.g., the right ventricle. The safety pacing energy of a secondheart chamber, e.g., the left ventricle, may be adjusted based on thecapture threshold of the first heart chamber.

Safety pacing pulses may be delivered 130, for example during a capturethreshold test or after suspension of an automatic capture verificationfeature.

Capture threshold tests are typically used to determine the capturethreshold. During the capture threshold test the pacing energy isstepped down until loss of capture is detected. Safety backup pacesfollow the primary paces of a capture threshold test to ensure thatthere is no interruption in therapy. Capture threshold testing may occuronce an hour or once a day, for example.

Those skilled in the art will appreciate that reference to a capturethreshold procedure indicates a method of determining the capturethreshold in one of left atrium, right atrium, left ventricle, rightventricle, or in any combination of heart chambers, e.g., left and rightatria and/or left and right ventricles. In such a procedure, thepacemaker, automatically or upon command, initiates a search for thecapture threshold of the selected heart chamber or chambers.

In one example of an automatic capture threshold procedure, thepacemaker delivers a sequence of pacing pulses to the heart and detectsthe cardiac responses to the pace pulses. The energy of the pacingpulses may be decreased in discrete steps until a predetermined numberof loss-of-capture events occur. The pacemaker may increase thestimulation energy in discrete steps until a predetermined number ofcapture events occur to confirm the capture threshold. A capturethreshold test may be performed using adaptive safety back up pacingmethods of the present invention.

Other procedures for implementing capture threshold testing may beutilized. In one example, the pacing energy may be increased in discretesteps until capture is detected. In another example, the pacing energymay be adjusted according to a binomial search pattern.

Automatic capture threshold determination is distinguishable fromautomatic capture verification, a procedure that may occur on abeat-by-beat basis during pacing. Automatic capture verificationverifies that a delivered pace pulse results in a captured response.When a captured response is not detected following a pace pulse, thepacemaker may deliver a back up safety pacing pulse to ensure consistentpacing. The back up safety pace may be delivered, for example, about70-80 ms after the initial pace pulse. If a predetermined number of pacepulses delivered during normal pacing do not produce a capturedresponse, the pacemaker may initiate several optional features.According to one optional feature, for example, if loss of capture isdetected, the pacemaker may schedule a capture threshold test tore-evaluate the capture threshold. Alternatively or additionally, thepacemaker may suspend automatic capture verification and revert to asafety pacing protocol that involves delivering pacing pulses at anenergy level that ensures capture and avoids interruption of pacingtherapy.

The embodiments of the present system illustrated herein are generallydescribed as being implemented in an implantable cardiac defibrillator(ICD) that may operate in numerous pacing modes known in the art.Various types of single and multiple chamber implantable cardiacdefibrillators are known in the art and may be used in connection withthe adaptive safety back up pacing methods of the present invention. Themethods of the present invention may also be implemented a variety ofimplantable or patient-external cardiac rhythm management devices,including single and multi chamber pacemakers, defibrillators,cardioverters, bi-ventricular pacemakers, and cardiac resynchronizers,for example.

Although the present system is described in conjunction with animplantable cardiac defibrillator having a microprocessor-basedarchitecture, it will be understood that the implantable cardiacdefibrillator (or other device) may be implemented in any logic-basedintegrated circuit architecture, if desired.

Referring now to FIG. 2 of the drawings, there is shown a cardiac rhythmmanagement system that may be used to implement adaptive safety back uppacing methods of the present invention. The cardiac rhythm managementsystem in FIG. 2 includes an ICD 200 electrically and physically coupledto a lead system 202. The housing and/or header of the ICD 200 mayincorporate a can electrode 309 used to provide electrical stimulationenergy to the heart and to sense cardiac electrical activity. The ICD200 may utilize all or a portion of the ICD housing as a can electrode309.

The lead system 202 is used to detect electric cardiac signals producedby the heart 201 and to provide electrical energy to the heart 201 undercertain predetermined conditions to treat cardiac arrhythmias. The leadsystem 202 may include one or more electrodes used for pacing, sensing,and/or defibrillation. In the embodiment shown in FIG. 2, the leadsystem 202 includes an intracardiac right ventricular (RV) lead system204, an intracardiac right atrial (RA) lead system 205, an intracardiacleft ventricular (LV) lead system 206. The lead system 202 of FIG. 2illustrates one embodiment that may be used in connection with theadaptive safety back up pacing methodologies described herein. Otherleads and/or electrodes may additionally or alternatively be used.

The lead system 202 may include intracardiac leads 204, 205, 206implanted in a human body with portions of the intracardiac leads 204,205, 206 inserted into a heart 201. The intracardiac leads 204, 205, 206include various electrodes positionable within the heart for sensingelectrical activity of the heart and for delivering electricalstimulation energy to the heart, for example, pacing pulses and/ordefibrillation shocks to treat various arrhythmias of the heart.

Other configurations of endocardial leads and electrodes are possibleand are considered to be included within the scope of the invention. Theother configurations of endocardial leads and electrodes may be used toimplement sensing and/or pacing in any one or more of the right atrium,right ventricle, left atrium and left ventricle. Although notillustrated in FIG. 2, the lead system 202 may additionally oralternatively include one or more extracardiac leads having electrodes,e.g., epicardial electrodes, positioned at locations outside the heartfor sensing and pacing one or more heart chambers including any one ormore of the right atrium, right ventricle, left atrium and leftventricle.

The right ventricular lead system 204 illustrated in FIG. 2 includes anSVC-coil 216, an RV-coil 214, an RV-ring electrode 211, and an RV-tipelectrode 212. The right ventricular lead system 204 extends through theright atrium 220 and into the right ventricle 218. In particular, theRV-tip electrode 212, RV-ring electrode 211, and RV-coil electrode 214are positioned at appropriate locations within the right ventricle 218for sensing and delivering electrical stimulation pulses to the heart.The SVC-coil 216 is positioned at an appropriate location within theright atrium chamber 220 of the heart 201 or a major vein leading to theright atrial chamber 220 of the heart 201.

In one configuration, the RV-tip electrode 212 referenced to the canelectrode 309 may be used to implement unipolar pacing and/or sensing inthe right ventricle 219. Bipolar pacing and/or sensing in the rightventricle may be implemented using the RV-tip 212 and RV-ring 211electrodes. In yet another configuration, the RV-ring 211 electrode mayoptionally be omitted, and bipolar pacing and/or sensing may beaccomplished using the RV-tip electrode 212 and the RV-coil 214, forexample. The right ventricular lead system 204 may be configured as anintegrated bipolar pace/shock lead. The RV-coil 214 and the SVC-coil 216are defibrillation electrodes.

The left ventricular lead 206 includes an LV distal electrode 213 and anLV proximal electrode 217 located at appropriate locations in or aboutthe left ventricle 224 for pacing and/or sensing the left ventricle 224.The left ventricular lead 206 may be guided into the right atrium 220 ofthe heart via the superior vena cava. From the right atrium 220, theleft ventricular lead 206 may be deployed into the coronary sinusostium, the opening of the coronary sinus 250. The lead 206 may beguided through the coronary sinus 250 to a coronary vein of the leftventricle 224. This vein is used as an access pathway for leads to reachthe surfaces of the left ventricle 224 which are not directly accessiblefrom the right side of the heart. Lead placement for the leftventricular lead 206 may be achieved via subclavian vein access and apreformed guiding catheter for insertion of the LV electrodes 213, 217adjacent to the left ventricle.

Unipolar pacing and/or sensing in the left ventricle may be implemented,for example, using the LV distal electrode referenced to the canelectrode 309. The LV distal electrode 213 and the LV proximal electrode217 may be used together as bipolar sense and/or pace electrodes for theleft ventricle. The left ventricular lead 206 and the right ventricularlead 204, in conjunction with the ICD 200, may be used to providecardiac resynchronization therapy such that the ventricles of the heartare paced substantially simultaneously, or in phased sequence, toprovide enhanced cardiac pumping efficiency for patients suffering fromchronic heart failure.

The right atrial lead 205 includes a RA-tip electrode 256 and an RA-ringelectrode 254 positioned at appropriate locations in the right atrium220 for sensing and pacing the right atrium 220. In one configuration,the RA-tip 256 referenced to the can electrode 309, for example, may beused to provide unipolar pacing and/or sensing in the right atrium 220.In another configuration, the RA-tip electrode 256 and the RA-ringelectrode 254 may be used to effect bipolar pacing and/or sensing.

Referring now to FIG. 3, there is shown an embodiment of a cardiacdefibrillator 300 suitable for implementing an adaptive safety back uppacing methodology of the present invention. FIG. 3 shows a cardiacdefibrillator divided into functional blocks. It is understood by thoseskilled in the art that there exist many possible configurations inwhich these functional blocks can be arranged. The example depicted inFIG. 3 is one possible functional arrangement. Other arrangements arealso possible. For example, more, fewer or different functional blocksmay be used to describe a cardiac defibrillator suitable forimplementing the adaptive safety back up pacing methodology of thepresent invention. In addition, although the cardiac defibrillator 300depicted in FIG. 3 contemplates the use of a programmablemicroprocessor-based logic circuit, other circuit implementations may beutilized.

The cardiac defibrillator 300 depicted in FIG. 3 includes circuitry forreceiving cardiac signals from a heart and delivering electricalstimulation energy to the heart in the form of pacing pulses ordefibrillation shocks. In one embodiment, the circuitry of the cardiacdefibrillator 300 is encased and hermetically sealed in a housing 301suitable for implanting in a human body. Power to the cardiacdefibrillator 300 is supplied by an electrochemical battery 380. Aconnector block (not shown) is attached to the housing 301 of thecardiac defibrillator 300 to allow for the physical and electricalattachment of the lead system conductors to the circuitry of the cardiacdefibrillator 300.

The cardiac defibrillator 300 may be a programmable microprocessor-basedsystem, including a control system 320 and a memory 370. The memory 370may store parameters for various pacing, defibrillation, and sensingmodes, along with other parameters such as capture thresholdinformation. Further, the memory 370 may store data indicative ofcardiac signals received by other components of the cardiacdefibrillator 300. The memory 370 may be used, for example, for storinghistorical EGM and therapy data. The historical data storage mayinclude, for example, data obtained from long term patient monitoringused for trending or other diagnostic purposes. Historical data, as wellas other information, may be transmitted to an external programmer unit390 as needed or desired.

The control system 320 and memory 370 may cooperate with othercomponents of the cardiac defibrillator 300 to control the operations ofthe cardiac defibrillator 300. The control system depicted in FIG. 3incorporates a processor 325 for classifying cardiac responses to pacingstimulation and safety pacing protocols in accordance with variousembodiments of the present invention. The control system 320 may includeadditional functional components including a pacemaker control circuit322, an arrhythmia detector 321, capture threshold detection circuitry325, safety pacing energy control circuitry 323, along with othercomponents for controlling the operations of the cardiac defibrillator300.

Telemetry circuitry 360 may be implemented to provide communicationsbetween the cardiac defibrillator 300 and an external programmer unit390. In one embodiment, the telemetry circuitry 360 and the programmerunit 390 communicate using a wire loop antenna and a radio frequencytelemetric link, as is known in the art, to receive and transmit signalsand data between the programmer unit 390 and the telemetry circuitry360. In this manner, programming commands and other information may betransferred to the control system 320 of the cardiac defibrillator 300from the programmer unit 390 during and after implant. In addition,stored cardiac data pertaining to capture threshold, capture detectionand/or adaptive safety back up pacing, for example, along with otherdata, may be transferred to the programmer unit 390 from the cardiacdefibrillator 300.

In the embodiment of the cardiac defibrillator 300 illustrated in FIG.3, electrodes RA-tip 256, RA-ring 254, RV-tip 213, RV-ring 211, RV-coil,SVC-coil, LV distal electrode 213, LV proximal electrode 217, and canelectrode 309 are coupled through a switch matrix 310 to sensingcircuits 331, 332, 336, and 337.

A right atrial sensing circuit 331 serves to detect and amplifyelectrical signals from the right atrium of the heart. Bipolar sensingin the right atrium may be implemented, for example, by sensing voltagesdeveloped between the RA-tip 256 and the RA-ring 254. Unipolar sensingmay be implemented, for example, by sensing voltages developed betweenthe RA-tip 256 and the can electrode 309. Outputs from the right atrialsensing circuit are coupled to the control system 320.

A right ventricular sensing circuit 332 serves to detect and amplifyelectrical signals from the right ventricle of the heart. The rightventricular sensing circuit 332 may include, for example, a rightventricular rate channel 333 and a right ventricular shock channel 334.Right ventricular cardiac signals sensed through use of the RV-tip 213electrode are right ventricular near-field signals and are denoted RVrate channel signals. A bipolar RV rate channel signal may be sensed asa voltage developed between the RV-tip 213 and the RV-ring.Alternatively, bipolar sensing in the right ventricle may be implementedusing the RV-tip electrode 213 and the RV-coil 214. Unipolar ratechannel sensing in the right ventricle may be implemented, for example,by sensing voltages developed between the RV-tip 213 and the canelectrode 309.

Right ventricular cardiac signals sensed through use of defibrillationelectrodes 214, 216, 309 are far-field signals, also referred to as RVmorphology or RV shock channel signals. More particularly, a rightventricular shock channel signal may be detected as a voltage developedbetween the RV-coil 214 and the SVC-coil 216. A right ventricular shockchannel signal may also be detected as a voltage developed between theRV-coil 214 and the can electrode 309. In another configuration the canelectrode 309 and the SVC-coil electrode 216 may be electrically shortedand a RV shock channel signal may be detected as the voltage developedbetween the RV-coil 214 and the can electrode 309/SVC-coil 216combination.

Outputs from the right ventricular sensing circuit 332 are coupled tothe control system 320. Cardiac signals sensed using one or both of therate channel 333 and the shock channel 334 sensing amplifies may be usedby the arrhythmia detector 321 to detect arrhythmic cardiac rhythms.

A left ventricular sensing circuit 336 serves to detect and amplifyelectrical signals from the left ventricle of the heart. Bipolar sensingin the left ventricle may be implemented, for example, by sensingvoltages developed between the LV distal electrode 213 and the LVproximal electrode 217. Unipolar sensing may be implemented, forexample, by sensing voltages developed between the LV distal electrode213 or the LV proximal electrode 217 to the can electrode 309. Signalsdetected using combinations of the LV electrodes, 213, 217, and/or can309 electrodes may be sensed and amplified by the left ventricularsensing circuitry 336. The output of the left ventricular sensingcircuit 336 is coupled to the control system 320.

The outputs of the switching matrix 310 may be operated to coupleselected combinations of electrodes 211, 212, 213, 214, 216, 217, 256,254 to the captured response sensing circuit 337. The captured responsesensing circuit 337 serves to sense and amplify voltages developed usingvarious combinations of electrodes. Signals sensed by the capturedresponse sense circuit 337 are used by the capture threshold detector todetermine capture thresholds associated with various electrodeconfigurations.

Capture threshold information acquired by the capture thresholddetection circuit 325 may be used by the safety pacing energy controllerto control the energy of safety pacing pulses delivered to the heart.According to one embodiment, safety pacing pulses may be delivered usingthe adapted safety pacing energy after each of the primary pacing pulsesof a capture threshold test. According to another embodiment, safetypacing pulses may be delivered using the adapted safety pacing energyfollowing suspension of automatic capture verification.

Embodiments of the invention are directed to adjusting the safety pacingenergy if automatic capture verification is suspended. Conventionalapproaches pace at a set high pacing energy upon suspension of automaticcapture verification. The high pacing energy utilized by conventionalapproaches may decrease battery lifetime and/or cause patientdiscomfort.

To mitigate pacing discomfort and decreased battery life caused by usingan arbitrary high pacing energy during automatic capture verificationsuspension, the embodiments described herein utilize an algorithm thatautomatically adjusts and applies an appropriately lower pacingamplitude. Pacing discomfort can arise during RA or RV unipolar pacingif pace pulse energy is high enough that anodal stimulation of muscleproximal to the pulse generator case occurs. Similarly, LV pacing cancause patient discomfort if the pace energy is high enough that thephrenic nerve is stimulated.

The pacing amplitude is determined based on pacing threshold history. Insome embodiments, the adjusted pacing amplitude varies linearly withcapture threshold within a fixed range. Regardless of the calculation,the pacing amplitude may be subject to minimum and maximum limits. Ifrecent or current capture threshold tests indicate low pacingthresholds, the algorithm will apply a lower suspension mode pacingamplitude rather than an arbitrary fixed amplitude as in conventionalapproaches. If recent or current capture threshold tests indicate ahigher pacing threshold, the algorithm will apply an appropriatelyhigher suspension pacing amplitude based on capture thresholds measuredprior to entering suspension mode.

FIG. 4 is a flowchart illustrating a method for delivering safety pacingupon suspension of automatic capture verification in accordance withembodiments of the invention. When operating 410 in automatic captureverification mode, the pacemaker delivers 415 scheduled pacing pulsesand automatically determines if the pacing pulses capture the heart. Ifa sufficient number of pacing pulses do not produce capture, forexample, about 4 out of about 6 beats, the pacemaker may detect 420 lossof capture.

If loss of capture is detected 420, the pacemaker suspends 425 automaticcapture verification. If capture threshold information is available 435and was recently measured, for example, within about 24 hours, then therecently acquired capture threshold is used to adjust 440 the safetypacing energy. In one implementation, the safety pacing energy isadjusted to be a multiple of the capture threshold, for example, twicethe capture threshold.

In some implementations, the adjustment of the pacing energy is subjectto a minimum value, for example, about 3.5 volts, and a maximum value,for example, about 5 volts. In cases where the capture threshold is low,such as less than about 1 volt, the suspension mode pacing voltage wouldbe set to a minimum value, such as about 3.5 volts. In cases wherein thecapture threshold is higher, such as approaching about 2.5 volts, thepacing voltage would be set to a maximum value, such as about 5 volts.

If capture threshold information is not available 435 or was notrecently measured, then the pacing energy for the safety pacing mode isadjusted 445 to a predetermined value, for example about 5 volts.

After adjustment 440, 445 of the pacing energy, the pacemaker enters 450safety pacing mode. During safety pacing mode, beat-by-beat captureverification no longer occurs. Pacing pulses are delivered 455 using theadjusted safety pacing energy.

It may be possible that some patients may have automatic captureverification suspended for long periods of time depending on thepatient's suitability to automatic capture verification. The intelligentsafety pacing adjustment algorithm described herein will lessen thenegative impact of automatic capture verification suspension on batterylife. Further, using adaptive safety pacing adjustment will reducepatient discomfort associated with high pacing energies.

FIG. 5 is a flowchart illustrating a method of performing a capturethreshold test using an adjustable safety pacing energy in accordancewith embodiments of the invention. Upon initiation 510 of a capturethreshold test procedure, the system determines if automatic captureverification is active 515. If automatic capture verification is active515 at the start of the capture threshold test procedure, then thesafety pacing energy may be adjusted 520 to a predetermined amount abovethe capture threshold. For example the safety pacing voltage may be setto about 1.5 volts above the measured capture threshold. If the adjustedsafety pacing voltage is below a minimum value, such as about 3.5 volts,the safety pacing voltage may be set to the minimum value. If theadjusted safety pacing voltage is above a maximum value, such as about 5volts, the safety pacing voltage may be set to the maximum value.

If automatic capture verification mode is not active 515, then thesafety pacing energy is adjusted 530 to the energy level used forautomatic capture verification suspension mode. The capture thresholdtest begins 540 during which safety pacing pulses are delivered 550using the adjusted pacing energy.

The safety pacing adjustment algorithm for capture threshold testingprovides a minimum pacing energy, e.g., about 3.5 volts, for patientswith low capture thresholds, such as less than about 1 volt. Forpatients with high capture thresholds, setting the safety pacing energyto a predetermined value above the capture threshold provides anadequate stimulation level to ensure consistent capture.

A number of the examples presented herein involve block diagramsillustrating functional blocks used for in accordance with embodimentsof the present invention. It will be understood by those skilled in theart that there exist many possible configurations in which thesefunctional blocks can be arranged and implemented. The examples depictedherein provide examples of possible functional arrangements used toimplement the approaches of the invention. The components andfunctionality depicted as separate or discrete blocks/elements in thefigures in general can be implemented in combination with othercomponents and functionality. The depiction of such components andfunctionality in individual or integral form is for purposes of clarityof explanation, and not of limitation. It is also understood that thecomponents and functionality depicted in the Figures and describedherein can be implemented in hardware, software, or a combination ofhardware and software.

Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

1. A method of operating a cardiac device, comprising: deliveringnon-test pacing pulses to a heart in an automatic capture verification(ACV) mode; suspending the ACV mode in response to detecting loss ofcapture during a time the ACV mode is being implemented; initiating acapture threshold test (CTT); determining whether or not the ACV modewas active or inactive prior to initiation of the CTT; adjusting anenergy used for back-up pacing during the CTT in response to whether theACV mode was active or suspended prior to initiation of the CTT; anddelivering the back-up pacing during the CTT using the adjusted energy.2. The method of claim 1, further comprising: determining a capturethreshold; and if the ACV mode was active prior to initiation of theCTT, adjusting the energy used for back-up pacing to a predeterminedamount above the capture threshold.
 3. The method of claim 1, furthercomprising: if the ACV mode is suspended prior to initiation of the CTT,adjusting the energy used for back-up pacing based on a pacing energyused while the ACV mode is suspended.
 4. The method of claim 1, whereinadjusting the energy used for the back-up pacing during the CTTcomprises adjusting the energy for the back-up pacing during the CTT toa default value if the adjusted back-up pacing energy is below athreshold.
 5. The method of claim 1, wherein: determining a capturethreshold of a first heart chamber; and adjusting the energy for theback-up pacing in a second heart chamber based on the capture thresholdof the first heart chamber.
 6. A medical device, comprising: a pluralityof electrodes configured to electrically couple to a heart; a pacingcircuit coupled to the electrodes and configured to deliver non-testpacing pulses to a heart in an automatic capture verification (ACV)mode; a processor configured to: suspend the ACV mode in response todetecting loss of capture during a time the ACV mode is beingimplemented; initiate a capture threshold test (CTT); determine whetheror not the ACV mode was active or inactive prior to initiation of theCTT; and adjust an energy used for back-up pacing during the CTT inresponse to whether the ACV mode was active or suspended prior toinitiation of the CTT, wherein the pacing circuit is further configuredto deliver the back-up pacing during the CTT using the adjusted energy.7. The medical device of claim 6, wherein the processor is furtherconfigured to: determine a capture threshold; and if the ACV mode wasactive prior to initiation of the CTT, adjust the energy for the back-uppacing to a predetermined amount above the capture threshold.
 8. Themedical device of claim 6, wherein the processor is further configuredto adjust the energy used for the back-up pacing during the CTT based ona pacing energy used while the ACV is suspended if the ACV mode wassuspended prior to initiation of the CTT.
 9. The medical device of claim6, wherein the processor is further configured to adjust the energy forthe back-up pacing during the CTT to a default value if the adjustedsafety pacing energy is below a threshold.
 10. The medical device ofclaim 6, wherein the processor is further configured to: determine acapture threshold of a first heart chamber; and adjust the energy forthe back-up pacing during CTT in a second heart chamber based on thecapture threshold of the first heart chamber.
 11. The medical device ofclaim 6, wherein the processor is configured to determine if capturethreshold information is available or unavailable and to adjust theback-up pacing energy used during the CTT to a value based on thecapture threshold information if the capture threshold information isavailable.
 12. The medical device of claim 11, wherein the value is amultiple of a recently measured capture threshold.
 13. The medicaldevice of claim 6, wherein the processor is configured to determine if acapture threshold has been recently measured and to adjust the pacingenergy used during the CTT to the recently measured capture threshold.14. The medical device of claim 6, wherein the processor is configuredto determine if capture threshold information is available orunavailable and to adjust the back-up pacing energy used during the CTTto a predetermined value if the capture threshold information isunavailable.
 15. A system, comprising: a plurality of electrodesconfigured to electrically couple to a heart; means for suspending theACV mode in response to detecting loss of capture during a time the ACVmode is being implemented; means for initiating a capture threshold test(CTT); means for determining whether or not the ACV mode was active orinactive prior to initiation of the CTT; means for adjusting an energyused for back-up pacing during the CTT in response to whether the ACVmode was active or suspended prior to initiation of the CTT; and apacing circuit configured to deliver the back-up pacing pulses duringthe CTT using the adjusted energy.
 16. The system of claim 11, furthercomprising means for adjusting the energy for the back-up pacing to apredetermined amount above a capture threshold if the ACV mode wasactive prior to initiation of the CTT.
 17. The system of claim 11,further comprising means for adjusting the energy for used for theback-up pacing based on a pacing energy used while the ACV is suspendedif the ACV mode was suspended prior to initiation of the CTT.